Measurement of soil pH: Problems and solutions

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Abstract
PH measurements in soil systems present unique challenges in terms of the interpretation of the values obtained. The principles behind the glass/calomel electrode system are discussed as a backdrop to pH measurements in both pure solutions and soils. The influence of the liquid junction potential and salt concentration on the pH values of soil water suspensions are discussed in detail from which it emerges that the current practice of measuring soil pH in stirred soil suspensions is likely to result in the greatest errors being incurred. This is due to the large liquid junction potential of uncertain magnitude developed in such systems. Measurements in salt solutions, such as M KCl and 0.1 M CaCl2, reduce the magnitude of the liquid junction potential substantially and make the pH values obtained more reproducible and consistent. The position of the calomel electrode salt bridge is crucial in measuring soil pH and should always be positioned in the clear supernatant solution.

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